1. Field of the Invention
The present invention relates to an automobile driving torque distribution control system for controlling a torque distribution between front and rear drive wheels, and specifically to a control system for four-wheel drive vehicles which system is capable of determining a driving torque distribution ratio between front and rear drive wheels depending on a wheel speed difference or a revolution speed difference between the front and rear road wheels.
2. Description of the Prior Art
There have been proposed and developed various four-wheel drive vehicles with a driving torque distribution controller in which a driving torque distribution ratio between front and rear drive wheels can be variably controlled depending on driving conditions of the vehicle. As is generally known, there are two types of four-wheel drive vehicles, one being a front-engine, rear-wheel drive base mode four-wheel drive vehicle which operates usually at a rear-wheel drive mode, and the other being a front-engine, front-wheel drive base mode four-wheel drive vehicle which operates usually at a front-wheel drive mode. Such a conventional driving torque distribution control system traditionally includes a variable driving-torque adjustment device, for example, a variable torque clutch mechanism, a differential limiting device such as a limited slip center differential, or the like, which device is placed in a power train between front and rear drive wheels for variably adjusting a driving torque distribution ratio between the front and rear wheels by varying its clutch engaging force to a desired value. Two general types of variable torque clutch mechanisms are used: hydraulic pressure operated clutch and electromagnetic clutch. The former acts to vary a clutch engaging force by way of adjustment of a fluid pressure applied to its clutch piston, while the latter acts to vary frictional forces between clutch plates by way of adjustment of a value of exciting current applied to its proportional solenoid. One such conventional driving torque distribution control system has been disclosed in Japanese Patent Provisional Publication No. 1-204826. In general, when engine power is carried from the power train to drive wheels, there is some loss of traction owing to characteristics of tires. In particular, traction loss or slip loss is great during acceleration of the vehicle, as compared with during constant-speed driving. On the driving torque distribution control system of the Japanese Patent Provisional Publication No. 1-204826, in case of front-engine, rear-wheel drive base mode four-wheel drive vehicles, during constant-speed driving, a driving torque distribution ratio between front and rear road wheels is designed to be set to a particular ratio of 0:100% according to which the entire driving torque is transmitted only to the rear drive wheels. On the other hand, when there is a great wheel speed difference or great revolution speed difference between the front and rear wheels, for example, during acceleration, i.e., in case that the difference obtained by subtracting one detected wheel speed of the auxiliary drive wheel (front wheel) from the other detected wheel speed of the main drive wheel (rear wheel) is positive, the torque distribution controller determines that acceleration slip occurs at the main drive wheels (rear wheels) owing to excessive application of driving torque thereto, irrespective of tire characteristics, road conditions such as a coefficient of friction of a road surface, and cornering conditions such as a steering angle. Depending on the wheel speed difference or the wheel revolution speed difference, the controller calculates a target torque distribution ratio and generates a control signal based on the target torque distribution ratio to the above-noted variable driving torque adjustment device. The driving-torque adjustment device is responsive to the control signal to transfer a designated amount of driving torque from the main drive wheels to the auxiliary drive wheels, thereby suppressing wheel spin (acceleration slip) at the rear drive wheels. On front-engine, rear-drive base mode four-wheel drive vehicles, the reasons for usually setting the driving torque distribution ratio between front and rear road wheels to 0:100% are described hereinafter.
As is generally known, the greater the distribution of driving force applied to the auxiliary drive wheels (front wheels), the more a driving stability of the vehicle is enhanced. In other words, as the distribution ratio of driving force is shifted from 0:100% to 50%:50%, the drivability or running stability can be improved. However, in such a case, fuel consumption may be degraded. In order to improve fuel consumption, the driving-torque distribution ratio between the auxiliary drive wheel and the main drive wheel is generally set to 0:100% in the event that there is less wheel speed difference or less wheel revolution speed, for instance, during usual running of the vehicle such as constant-speed driving.
The controller of the previously-noted conventional driving torque distribution control system traditionally utilizes a predetermined data map to calculate or derive a desired driving-torque distribution ratio depending on the wheel speed difference. For the purpose of enhancing a responsiveness of the driving-torque distribution control, the data map usually consists of a particular driving torque characteristic curve, in which a distribution of driving torque transmitted to the auxiliary drive wheel is simply increased from an origin (a point at which the wheel speed difference is zero) in accordance with an increase in the detected wheel speed difference. In the previously-noted conventional driving torque distribution control system, there is some problem as explained below.
For instance, in four-wheel drive vehicles with a manual transmission, in the event that engagement and disengagement of the clutch are repeatedly performed to shift up during acceleration, transmission of driving torque to the drive wheels is temporarily stopped during shifting. Thus, the drive wheels tend to roll freely coastingly on the roads with less resistance due to inertia of the vehicle during shifting. Assuming that the outside diameter of the front tires are equal to that of the rear tires, the detected wheel speed difference or wheel revolution speed difference becomes temporarily to zero owing to coasting of each wheel. The vehicle could experience a rapid change of the wheel speed difference down to zero during undesired depression of the clutch pedal accidentally. Also, in four-wheel drive vehicles with an automatic transmission, the vehicle could experience such a rapid change of the wheel speed difference down to zero during automatic shifting operation. In the above-noted cases, the conventional torque control system would determine a driving torque distribution ratio based on the above-explained simple torque characteristic curve memorized in the storage of the controller in the form of a data map, so that the driving-torque distribution ratio between the auxiliary drive wheel and the main drive wheel is set to 0:100% due to the wheel speed difference of zero. In general, since the driving-torque control system consists of a feedback control system based on the detected speed difference or wheel revolution speed difference, there is a delay of response time in the system until the system outputs a newly derived control signal after a predetermined operation time (sampling time) has elapsed. Additionally, there is a delay of mechanical transmission in an actuator included in the driving-torque adjustment device such as a variable torque clutch mechanism or a differential limiting device, after outputting the control signal. Even if the actuator is responsive quickly to the control signal from the control system at real-time, there is a delay of response time owing to the predetermined operation time of the system. Hence, upon the torque distribution ratio between the auxiliary drive wheel (front wheel) and the main drive wheel (rear wheel) is switched quickly to 0:100% during shifting, the wheel speed difference is rapidly increased with a considerable up-gradient (at a steep rate of change in the wheel speed difference), owing to a rapid increase in the wheel speed at the main drive wheels. When the clutch is engaged again after completion of shifting operation, the entire driving torque is transferred to the main drive wheel (rear wheel). There is a possibility that such a rapid change in driving torque causes the rear wheel to slip. In order to suppress such a relatively great wheel speed difference of the front and rear wheels and to adjust the driving-torque distribution to an optimal driving torque distribution condition, the controller derives a new distribution ratio based on the newly detected wheel speed difference. The torque-distribution control signal representative of the new distribution ratio is also generated with the predetermined time delay. In other words, the derived distribution ratio varies at a gradient or control gain based on the predetermined time delay of the system. It will be appreciated that the greater the response-time delay of the system the smaller the control gain. Owing to the additional delay of mechanical transmission of the actuator, there is a possibility that the control gain of the control system can be adjusted to a more smaller value. Due to the delay time, the responsiveness of the conventional torque-distribution control system would be degraded. As is generally known, under the above-mentioned optimal torque-distribution condition, the main drive wheels rotate slightly faster than the auxiliary drive wheels with an optimal slippage (an optimal wheel speed difference) which is determined depending on several factors, such as a car weight, a road surface condition, kinds of tires or the like. As explained previously, in the event that excessive driving torque is rapidly applied to the main drive wheels for a short time, the wheel speed difference between the main drive wheel and the auxiliary drive wheel is rapidly increased Thus, this may result in a great overshoot relative to the optimal slippage at the main drive wheels. Such overshoot causes a great energy loss resulting from slip loss or traction loss. A time duration or interval in which the detected wheel speed difference overshot the optimal wheel speed difference is generally called as a slip-loss time. The energy loss resulting from the undesirable slippage (acceleration-slip) results in degradation of fuel consumption. The energy loss is substantially equivalent to the integral of slip losses accumulated for the slip-loss time. Hence, the conventional torque distribution control system tends to adjust the front-and-rear driving torque distribution to the predetermined optimal distribution condition with a relatively long time interval of slip-loss, owing to the unnegligible time delay resulting from the operation time of the system. To avoid this, if a torque distribution ratio is held to the previous driving-torque distribution ratio already assigned by the control system before the vehicle experience rapid changes in the wheel speed difference, such system may induce another problem. For instance, in the event that the wheel speed difference is rapidly decreased, i.e., when the driving condition of the vehicle is rapidly shifted from an accelerating state to a constant-speed driving state, the slipping condition of the main drive wheels could be rapidly shifted from a great slip state to a less slip state, owing to the torque difference between a great driving torque consumed to accelerate the vehicle against running resistance including acceleration resistance during acceleration anal a small driving torque consumed to run the vehicle at a constant speed during constant-speed driving, with regard to the main drive wheels. Thus, the wheel speed difference could be reduced rapidly, when shifted from the accelerating state to the constant-speed driving state. As previously noted, assuming that a torque distribution ratio is held to the previous distribution ratio already assigned by the control system before a rapid change in the wheel speed difference, the distribution ratio of the auxiliary drive wheel to the main drive wheel is not set to zero (0:100%) but remains held to a relatively great value. In this case, fuel consumption may be generally degraded due to four-wheel drive tendencies.